Multi-stage slice recovery in a dispersed storage network

ABSTRACT

A method for use by a computing device in a dispersed storage network (DSN) to recover corrupt encoded data slices. In response to a request to storage units of the DSN for encoded data slices corresponding to a data segment, the computing device of a receives less than a decode threshold number of valid encoded data slices and at least one integrity error message that provides an indication of a corrupt encoded data slice. The computing device requests and receives at least one corrupt encoded data slice corresponding to the integrity error message(s). Utilizing at least one correction approach involving stored integrity data, the computing device then corrects the corrupt slice(s) to produce a decode threshold number of encoded data slices in order to decode the corresponding data segment. A variety of correction approaches may be employed, including a multi-stage approach that utilizes data from both valid and invalid slices.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

Technical Field of the Invention

This invention relates generally to computer networks and more particularly to dispersing and recovering error encoded data.

Description of Related Art

Computing devices are known to communicate data, process data, and/or store data. Such computing devices range from wireless smart phones, laptops, tablets, personal computers (PC), work stations, and video game devices, to data centers that support millions of web searches, stock trades, or on-line purchases every day. In general, a computing device includes a central processing unit (CPU), a memory system, user input/output interfaces, peripheral device interfaces, and an interconnecting bus structure.

As is further known, a computer may effectively extend its CPU by using “cloud computing” to perform one or more computing functions (e.g., a service, an application, an algorithm, an arithmetic logic function, etc.) on behalf of the computer. Further, for large services, applications, and/or functions, cloud computing may be performed by multiple cloud computing resources in a distributed manner to improve the response time for completion of the service, application, and/or function. For example, Hadoop is an open source software framework that supports distributed applications enabling application execution by thousands of computers.

In addition to cloud computing, a computer may use “cloud storage” as part of its memory system. As is known, cloud storage enables a user, via its computer, to store files, applications, etc. on a remote storage system. The remote storage system may include a RAID (redundant array of independent disks) system and/or a dispersed storage system that uses an error correction scheme to encode data for storage.

In a RAID system, a RAID controller adds parity data to the original data before storing it across an array of disks. The parity data is calculated from the original data such that the failure of a single disk typically will not result in the loss of the original data. While RAID systems can address certain memory device failures, these systems may suffer from effectiveness, efficiency and security issues. For instance, as more disks are added to the array, the probability of a disk failure rises, which may increase maintenance costs. When a disk fails, for example, it needs to be manually replaced before another disk(s) fails and the data stored in the RAID system is lost. To reduce the risk of data loss, data on a RAID device is often copied to one or more other RAID devices. While this may reduce the possibility of data loss, it also raises security issues since multiple copies of data may be available, thereby increasing the chances of unauthorized access. In addition, co-location of some RAID devices may result in a risk of a complete data loss in the event of a natural disaster, fire, power surge/outage, etc.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic block diagram of an embodiment of a dispersed or distributed storage network (DSN) in accordance with the present invention;

FIG. 2 is a schematic block diagram of an embodiment of a computing core in accordance with the present invention;

FIG. 3 is a schematic block diagram of an example of dispersed storage error encoding of data in accordance with the present invention;

FIG. 4 is a schematic block diagram of a generic example of an error encoding function in accordance with the present invention;

FIG. 5 is a schematic block diagram of a specific example of an error encoding function in accordance with the present invention;

FIG. 6 is a schematic block diagram of an example of a slice name of an encoded data slice (EDS) in accordance with the present invention;

FIG. 7 is a schematic block diagram of an example of dispersed storage error decoding of data in accordance with the present invention;

FIG. 8 is a schematic block diagram of a generic example of an error decoding function in accordance with the present invention;

FIGS. 9 and 10 are schematic block diagrams of an embodiment of a DSN in accordance with the present invention;

FIG. 11 is a logic diagram of an example of recovery of corrupted data in accordance with the present invention; and

FIG. 12 is a logic diagram of a further example of recovery of corrupted data in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of an embodiment of a dispersed, or distributed, storage network (DSN) 10 that includes a plurality of computing devices 12-16, a managing unit 18, an integrity processing unit 20, and a DSN memory 22. The components of the DSN 10 are coupled to a network 24, which may include one or more wireless and/or wire lined communication systems; one or more non-public intranet systems and/or public internet systems; and/or one or more local area networks (LAN) and/or wide area networks (WAN).

The DSN memory 22 includes a plurality of storage units 36 that may be located at geographically different sites (e.g., one in Chicago, one in Milwaukee, etc.), at a common site, or a combination thereof. For example, if the DSN memory 22 includes eight storage units 36, each storage unit is located at a different site. As another example, if the DSN memory 22 includes eight storage units 36, all eight storage units are located at the same site. As yet another example, if the DSN memory 22 includes eight storage units 36, a first pair of storage units are at a first common site, a second pair of storage units are at a second common site, a third pair of storage units are at a third common site, and a fourth pair of storage units are at a fourth common site. Note that a DSN memory 22 may include more or less than eight storage units 36. Further note that each storage unit 36 includes a computing core (as shown in FIG. 2, or components thereof) and a plurality of memory devices for storing dispersed error encoded data.

Each of the computing devices 12-16, the managing unit 18, and the integrity processing unit 20 include a computing core 26, which includes network interfaces 30-33. Computing devices 12-16 may each be a portable computing device and/or a fixed computing device. A portable computing device may be a social networking device, a gaming device, a cell phone, a smart phone, a digital assistant, a digital music player, a digital video player, a laptop computer, a handheld computer, a tablet, a video game controller, and/or any other portable device that includes a computing core. A fixed computing device may be a computer (PC), a computer server, a cable set-top box, a satellite receiver, a television set, a printer, a fax machine, home entertainment equipment, a video game console, and/or any type of home or office computing equipment. Note that each of the managing unit 18 and the integrity processing unit 20 may be separate computing devices, may be a common computing device, and/or may be integrated into one or more of the computing devices 12-16 and/or into one or more of the storage units 36.

Each interface 30, 32, and 33 includes software and hardware to support one or more communication links via the network 24 indirectly and/or directly. For example, interface 30 supports a communication link (e.g., wired, wireless, direct, via a LAN, via the network 24, etc.) between computing devices 14 and 16. As another example, interface 32 supports communication links (e.g., a wired connection, a wireless connection, a LAN connection, and/or any other type of connection to/from the network 24) between computing devices 12 and 16 and the DSN memory 22. As yet another example, interface 33 supports a communication link for each of the managing unit 18 and the integrity processing unit 20 to the network 24.

Computing devices 12 and 16 include a dispersed storage (DS) client module 34, which enables the computing device to dispersed storage error encode and decode data (e.g., data object 40) as subsequently described with reference to one or more of FIGS. 3-8. In this example embodiment, computing device 16 functions as a dispersed storage processing agent for computing device 14. In this role, computing device 16 dispersed storage error encodes and decodes data on behalf of computing device 14. With the use of dispersed storage error encoding and decoding, the DSN 10 is tolerant of a significant number of storage unit failures (the number of failures is based on parameters of the dispersed storage error encoding function) without loss of data and without the need for a redundant or backup copies of the data. Further, the DSN 10 stores data for an indefinite period of time without data loss and in a secure manner (e.g., the system is very resistant to unauthorized attempts at accessing the data).

In operation, the managing unit 18 performs DS management services. For example, the managing unit 18 establishes distributed data storage parameters (e.g., vault creation, distributed storage parameters, security parameters, billing information, user profile information, etc.) for computing devices 12-14 individually or as part of a group of user devices. As a specific example, the managing unit 18 coordinates creation of a vault (e.g., a virtual memory block associated with a portion of an overall namespace of the DSN) within the DSN memory 22 for a user device, a group of devices, or for public access and establishes per vault dispersed storage (DS) error encoding parameters for a vault. The managing unit 18 facilitates storage of DS error encoding parameters for each vault by updating registry information of the DSN 10, where the registry information may be stored in the DSN memory 22, a computing device 12-16, the managing unit 18, and/or the integrity processing unit 20.

The managing unit 18 creates and stores user profile information (e.g., an access control list (ACL)) in local memory and/or within memory of the DSN memory 22. The user profile information includes authentication information, permissions, and/or the security parameters. The security parameters may include encryption/decryption scheme, one or more encryption keys, key generation scheme, and/or data encoding/decoding scheme.

The managing unit 18 creates billing information for a particular user, a user group, a vault access, public vault access, etc. For instance, the managing unit 18 tracks the number of times a user accesses a non-public vault and/or public vaults, which can be used to generate per-access billing information. In another instance, the managing unit 18 tracks the amount of data stored and/or retrieved by a user device and/or a user group, which can be used to generate per-data-amount billing information.

As another example, the managing unit 18 performs network operations, network administration, and/or network maintenance. Network operations includes authenticating user data allocation requests (e.g., read and/or write requests), managing creation of vaults, establishing authentication credentials for user devices, adding/deleting components (e.g., user devices, storage units, and/or computing devices with a DS client module 34) to/from the DSN 10, and/or establishing authentication credentials for the storage units 36. Network administration includes monitoring devices and/or units for failures, maintaining vault information, determining device and/or unit activation status, determining device and/or unit loading, and/or determining any other system level operation that affects the performance level of the DSN 10. Network maintenance includes facilitating replacing, upgrading, repairing, and/or expanding a device and/or unit of the DSN 10.

To support data storage integrity verification within the distributed computing system 10, the integrity processing unit 20 (and/or other devices in the DSN 10) may perform rebuilding of ‘bad’ or missing encoded data slices. At a high level, the integrity processing unit 20 performs rebuilding by periodically attempting to retrieve/list encoded data slices, and/or slice names of the encoded data slices, from the DSN memory 22. Retrieved encoded slices are checked for errors due to data corruption, outdated versioning, etc. If a slice includes an error, it is flagged as a ‘bad’ or ‘corrupt’ slice. Encoded data slices that are not received and/or not listed may be flagged as missing slices. Bad and/or missing slices may be subsequently rebuilt using other retrieved encoded data slices that are deemed to be good slices in order to produce rebuilt slices. The rebuilt slices may then be written to DSN memory 22. Note that the integrity processing unit 20 may be a separate unit as shown, included in DSN memory 22, included in the computing device 16, and/or distributed among the storage units 36. As described more fully below in conjunction with FIGS. 9-12, a multi-stage decoding process may be employed in certain circumstances to recover data even when the number of valid encoded data slices of a set of encoded data slices is less than a relevant decode threshold number.

FIG. 2 is a schematic block diagram of an embodiment of a computing core 26 that includes a processing module 50, a memory controller 52, main memory 54, a video graphics processing unit 55, an input/output (IO) controller 56, a peripheral component interconnect (PCI) interface 58, an IO interface module 60, at least one 10 device interface module 62, a read only memory (ROM) basic input output system (BIOS) 64, and one or more memory interface modules. The one or more memory interface module(s) includes one or more of a universal serial bus (USB) interface module 66, a host bus adapter (HBA) interface module 68, a network interface module 70, a flash interface module 72, a hard drive interface module 74, and a DSN interface module 76.

The DSN interface module 76 functions to mimic a conventional operating system (OS) file system interface (e.g., network file system (NFS), flash file system (FFS), disk file system (DFS), file transfer protocol (FTP), web-based distributed authoring and versioning (WebDAV), etc.) and/or a block memory interface (e.g., small computer system interface (SCSI), internet small computer system interface (iSCSI), etc.). The DSN interface module 76 and/or the network interface module 70 may function as one or more of the interface 30-33 of FIG. 1. Note that the IO device interface module 62 and/or the memory interface modules 66-76 may be collectively or individually referred to as IO ports.

FIG. 3 is a schematic block diagram of an example of dispersed storage error encoding of data. When a computing device 12 or 16 has data to store it disperse storage error encodes the data in accordance with a dispersed storage error encoding process based on dispersed storage error encoding parameters. The dispersed storage error encoding parameters include an encoding function (e.g., information dispersal algorithm, Reed-Solomon, Cauchy Reed-Solomon, systematic encoding, non-systematic encoding, on-line codes, etc.), a data segmenting protocol (e.g., data segment size, fixed, variable, etc.), and per data segment encoding values. The per data segment encoding values include a total, or pillar width, number (T) of encoded data slices per encoding of a data segment (i.e., in a set of encoded data slices); a decode threshold number (D) of encoded data slices of a set of encoded data slices that are needed to recover the data segment; a read threshold number (R) of encoded data slices to indicate a number of encoded data slices per set to be read from storage for decoding of the data segment; and/or a write threshold number (W) to indicate a number of encoded data slices per set that must be accurately stored before the encoded data segment is deemed to have been properly stored. The dispersed storage error encoding parameters may further include slicing information (e.g., the number of encoded data slices that will be created for each data segment) and/or slice security information (e.g., per encoded data slice encryption, compression, integrity checksum, etc.).

In the present example, Cauchy Reed-Solomon has been selected as the encoding function (a generic example is shown in FIG. 4 and a specific example is shown in FIG. 5); the data segmenting protocol is to divide the data object into fixed sized data segments; and the per data segment encoding values include: a pillar width of 5, a decode threshold of 3, a read threshold of 4, and a write threshold of 4. In accordance with the data segmenting protocol, the computing device 12 or 16 divides the data (e.g., a file (e.g., text, video, audio, etc.), a data object, or other data arrangement) into a plurality of fixed sized data segments (e.g., 1 through Y of a fixed size in range of Kilo-bytes to Tera-bytes or more). The number of data segments created is dependent of the size of the data and the data segmenting protocol.

The computing device 12 or 16 then disperse storage error encodes a data segment using the selected encoding function (e.g., Cauchy Reed-Solomon) to produce a set of encoded data slices. FIG. 4 illustrates a generic Cauchy Reed-Solomon encoding function, which includes an encoding matrix (EM), a data matrix (DM), and a coded matrix (CM). The size of the encoding matrix (EM) is dependent on the pillar width number (T) and the decode threshold number (D) of selected per data segment encoding values. To produce the data matrix (DM), the data segment is divided into a plurality of data blocks and the data blocks are arranged into D number of rows with Z data blocks per row. Note that Z is a function of the number of data blocks created from the data segment and the decode threshold number (D). The coded matrix is produced by matrix multiplying the data matrix by the encoding matrix.

FIG. 5 illustrates a specific example of Cauchy Reed-Solomon encoding with a pillar number (T) of five and decode threshold number of three. In this example, a first data segment is divided into twelve data blocks (D1-D12). The coded matrix includes five rows of coded data blocks, where the first row of X11-X14 corresponds to a first encoded data slice (EDS 1_1), the second row of X21-X24 corresponds to a second encoded data slice (EDS 2_1), the third row of X31-X34 corresponds to a third encoded data slice (EDS 3_1), the fourth row of X41-X44 corresponds to a fourth encoded data slice (EDS 4_1), and the fifth row of X51-X54 corresponds to a fifth encoded data slice (EDS 5_1). Note that the second number of the EDS designation corresponds to the data segment number. In the illustrated example, the value X11=aD1+bD5+cD9, X12=aD2+bD6+cD10, . . . X53=mD3+nD7+oD11, and X54=mD4+nD8+oD12.

Returning to the discussion of FIG. 3, the computing device also creates a slice name (SN) for each encoded data slice (EDS) in the set of encoded data slices. A typical format for a slice name 80 is shown in FIG. 6. As shown, the slice name (SN) 80 includes a pillar number of the encoded data slice (e.g., one of 1−T), a data segment number (e.g., one of 1−Y), a vault identifier (ID), a data object identifier (ID), and may further include revision level information of the encoded data slices. The slice name functions as at least part of a DSN address for the encoded data slice for storage and retrieval from the DSN memory 22.

As a result of encoding, the computing device 12 or 16 produces a plurality of sets of encoded data slices, which are provided with their respective slice names to the storage units for storage. As shown, the first set of encoded data slices includes EDS 1_1 through EDS 5_1 and the first set of slice names includes SN 1_1 through SN 5_1 and the last set of encoded data slices includes EDS 1_Y through EDS 5_Y and the last set of slice names includes SN 1_Y through SN 5_Y.

FIG. 7 is a schematic block diagram of an example of dispersed storage error decoding of a data object that was dispersed storage error encoded and stored in the example of FIG. 4. In this example, the computing device 12 or 16 retrieves from the storage units at least the decode threshold number of encoded data slices per data segment. As a specific example, the computing device retrieves a read threshold number of encoded data slices.

In order to recover a data segment from a decode threshold number of encoded data slices, the computing device uses a decoding function as shown in FIG. 8. As shown, the decoding function is essentially an inverse of the encoding function of FIG. 4. The coded matrix includes a decode threshold number of rows (e.g., three in this example) and the decoding matrix in an inversion of the encoding matrix that includes the corresponding rows of the coded matrix. For example, if the coded matrix includes rows 1, 2, and 4, the encoding matrix is reduced to rows 1, 2, and 4, and then inverted to produce the decoding matrix.

FIGS. 9 and 10 are schematic block diagrams of an embodiment of a dispersed storage network (DSN) in accordance with the present invention. The illustrated DSN includes the computing device 16 of FIG. 1, the network 24 of FIG. 1, and a set of storage units 1-8. The computing device 16 includes the DS client module 34 of FIG. 1. Each storage unit 1-8 may be implemented utilizing the storage unit 36 of FIG. 1. The DSN functions to recover stored and segmented data, where a data segment is dispersed storage error encoded in accordance with dispersal parameters to produce a set of encoded data slices 1-8 for storage in the set of storage units 1-8, respectively. The dispersal parameters include a width (e.g., n) and a decode threshold number k, where at least a decode threshold number of encoded data slices of the set of n encoded data slices is required to recover the data segment. For example, eight storage units are utilized when the width is eight and five encoded data slices are required to recover the data segment when the decode threshold is five. Each encoded data slice includes a plurality of bytes b1-bB. As discussed more fully below, byte-by-byte recovery/rebuilding of the encoded data segment may be possible utilizing uncorrupted bytes in a common byte position from a decode threshold number of encoded data slices.

Referring more particularly to FIG. 9, in an exemplary operation of the recovery of stored data the DS client module 34 issues at least a decode threshold number of slice requests to the set of storage units to initiate retrieval of a decode threshold number of slices to enable recovery of the data segment. For example, the DS client module 34 issues, via the network 24, slice requests 1-8 to the memory units 1-8 in an attempt to recover at least the decode threshold number of encoded data slices of the set of encoded data slices 1-8. Each storage unit receiving a corresponding slice request recovers a corresponding requested encoded data slice from a local memory, and may perform an integrity test on the retrieved encoded data slice. In one example, the storage unit may compare a calculated integrity value to a stored integrity value. Potential corruption of the requested encoded data slice is indicated when the calculated integrity value compares unfavorably to the stored integrity value (e.g., not substantially the same).

The storage unit generates a slice response based on the integrity test. For example, storage unit 5 generates a slice response 5 that indicates that the encoded data slice 5 is likely corrupt when the corresponding integrity test indicates the potential corruption. In certain embodiments, the storage unit 5 may include the encoded data slice 5 in the slice response 5 in conjunction with an error indication. As another example, storage unit 4 generates a slice response 4 that includes the encoded data slice 4 when the corresponding integrity test indicates no potential corruption. In the illustrated embodiment, storage unit 8 is unavailable and does not respond to slice request 8 (which request may not be sent if the DS client module 34 has advance knowledge of the unavailability of storage unit 8).

As illustrated in FIG. 10, when receiving less than the decode threshold number of uncorrupted encoded data slices (e.g., the DS client module 34 receives encoded data slices 1-4) and an indication of at least one corrupted encoded data slice (e.g., slice responses 5-7 indicating that the encoded data slices 5-7 are available but are potentially corrupted), the DS client module 34 selects one or more potentially corrupt encoded data slices for retrieval. For example, the DS client module 34 may attempt to retrieve two corrupted slices beyond the relevant decode threshold number of encoded data slices for each corrupted slice of the decode threshold number of encoded data slices in order to attempt to correct a bad byte of an encoded data slice. For instance, the DS client module 34 determines to recover two corrupted encoded data slices beyond the decode threshold number of encoded data slices in accordance with a formula: (7−5)/2=1. In particular, the DS client module 34 selects corrupted encoded data slices 5-7 for retrieval. In this example, slice 5 includes corrupted byte b2, slice 6 includes corrupted byte b1, and slice 7 includes corrupted byte b3. As used herein, the terms“corrupt”, “corrupted”, “likely corrupt” and“potentially corrupted” are used interchangeably as contextually appropriate.

Having selected the one or more corrupted encoded data slices for retrieval, the DS client module 34 facilitates retrieval of the selected corrupted encoded data slices. For example, the DS client module 34 issues, via the network 24, corrupted slice requests 5-7 to the storage units 5-7 and receives, via the network 24, corrupted encoded data slices 5-7 from the storage units 5-7.

Having received the selected corrupted encoded data slices, the DS client module 34 corrects at least one corrupted encoded data slice using at least one correction approach to produce the decode threshold number of uncorrupted encoded data slices. For example, the DS client module 34 selects a first approach (e.g., byte-by-byte slice decoding or slice substitution) based on available integrity information, attempts correction utilizing the first correction approach, and applies another correction approach when necessary (e.g., slice substitution when a data segment integrity value is known by the DS client module 34). Having corrected the at least one corrupted encoded data slice to produce the decode threshold number of uncorrupted encoded data slices, the DS client module 34 dispersed storage error decodes the decode threshold number of uncorrupted encoded data slices to produce an error free data segment 90.

Integrity information may be generated and applied in a variety of ways in accordance with the present disclosure. Some approaches may prove to be more fruitful than others, especially when applied in certain orders, as the computational complexity, bandwidth and latency costs of different approaches can differ significantly. By way of example and without limitation, various“levels” at which integrity information can be applied include:

level 1—integrity check or error correct information is applied to the data source (e.g., using an All-Or-Nothing (AONT) known value, CRC, hash, digital signature, or check sum of data before the data is sliced);

level 2—integrity check or error correct information is applied as part of an error encoding function to produce slices (e.g., the normal error coding used to generate slices from a pre-encoded data segment);

level 3—integrity check or error correct information is applied to each slice before transmission to storage units for storage (TCP check sums, TLS authentication codes, hash lists or sliced hash lists); and

level 4—integrity check or error correct information is applied to each encoded data slice when stored by the storage unit (e.g., CRC/hash calculated and stored with the slice, error correction code (ECC) bits stored internally by file system or a memory device).

An integrity check failure can happen at various of such levels. For example, when reading an encoded data slice from a memory device, the storage unit may determine that the slice data does not match its corresponding stored integrity check value. This can result in an error being returned to the requester instead of the slice. However, in some cases the DS client module 34 may be able to recover partially corrupted encoded data slices by using inherently redundant information from other slices.

Accordingly, a special protocol request (e.g., a“ReadCorrupted” request) may be issued in response to reception of an integrity error following a slice request. Alternatively, a storage unit detecting a bad slice may return a read response including a slice and a flag indicating that the slice is likely corrupt (e.g., the storage unit may not be able to determine whether the data within the slice is corrupted or the integrity check value is wrong).

In most cases, absent any failure of“level 1” integrity check errors, the DS client module 34 may ignore slice-level integrity checks, and only rely on such checks should a level 2 or level 1 error occur. An example process for responding to different types of errors and performing data recovery in various ways is described below and in conjunction with FIG. 12.

FIG. 11 is a logic diagram of an example of recovery of corrupted data in accordance with the present invention. In particular, a method is presented for use in conjunction with one or more functions and features described in conjunction with FIGS. 1-10. The method begins at step 100 where a processing module (e.g., of a distributed storage (DS) client module) issues at least a decode threshold number of encoded data slice requests to storage units of a set of storage units to initiate retrieval of a decode threshold number of uncorrupted encoded data slices of a set of stored encoded data slices. For example, the processing module generates the at least the decode threshold number of encoded data slice requests to include corresponding slice names, identifies corresponding storage units, and sends the generated encoded data slice requests to the identified corresponding storage units.

When receiving less than the decode threshold number of uncorrupted encoded data slices and an indication of at least one corrupted encoded data slice, the processing module next (step 102) selects one or more corrupted encoded data slices for retrieval. For example, the processing module receives (e.g., in response to a request) two corrupted encoded data slices beyond the decode threshold number of encoded data slices for each corrupted encoded data slice of the decode threshold number of encoded data slices.

The method continues at the step 104 where the processing module retrieves the selected corrupted encoded data slices. For example, the processing module issues a corrupted slice requests (e.g., a “ReadCorrupted” request). The method continues at the step 106 where the processing module corrects at least one corrupted encoded data slice using at least one correction approach to produce the decode threshold number of uncorrupted encoded data slices. For example, the processing module selects a first approach based on available integrity information (e.g., byte-by-byte slice decoding or slice substitution), attempts the correction, and applies another approach (e.g., slice substitution when a data segment integrity value is known by the processing module) when necessary to produce the decode threshold number of uncorrupted encoded data slices. The method continues at the step 108 where the processing module dispersed storage error decodes the decode threshold number of uncorrupted encoded data slices to produce an error free data segment 90.

FIG. 12 is a logic diagram of a further example of recovery of corrupted data in accordance with the present invention. In particular, a method is presented for use in conjunction with one or more functions and features described in conjunction with FIGS. 1-10. The method begins at step 110 where a processing module (e.g., of a distributed storage (DS) client module) issues slice requests in an attempt to retrieve at least a decode threshold number of encoded data slices from the relevant storage units. If integrity errors (e.g., level 4 errors) occur, the processing module may attempt to read additional encoded data slices until at least a decode threshold number of slices are returned without integrity failures. When the decode threshold number of encoded data slices are not successfully retrieved (e.g., due to storage unit unavailability or an abundance of level 4 integrity errors), the processing module requests and receives at least one corrupt encoded data slice, even if the slice(s) manifest level 4 integrity errors (e.g., the processing module may use a ReadCorrupted request as necessary). Note that if fewer than a relevant decode threshold number of valid and corrupt encoded data slices are returned from the storage units, or if correction approaches are unsuccessful, a failure indication may be provided in response to the read request that initiated the method.

Next, in step 112, the processing module applies stored integrity information corresponding to the at least one corrupt data slice. For example, if level 3 integrity information is available for the encoded data slices (e.g., a slice hash list), then that information may be used to establish known integrity check values for all received slices. This integrity information may be used to determine if any slices presumed to be corrupt might instead be valid. If a decode threshold number of slices with matching level 3 integrity information are identified, then an attempt to dispersed storage error decode the slices may be performed (e.g., step 122). Successful decoding can be verified using level 2 and level 1 integrity information.

If the at least one corrupt encoded data slice is verified to be invalid, (or if level 2 or level 1 integrity failures occur), the method proceeds to step 114 and a first correction approach is selected based on additional stored integrity information. For example, if at least two or more slices beyond the relevant decode threshold are received, the processing module may attempt to use level 2 integrity information (e.g., encoding redundancy information) to recover single byte errors in each byte position of the received slices. An additional 2 slices beyond the decode threshold may be required to correct each error present in the corresponding byte position of a given slice. In one example in which n=8 and k=10, having 16 available slices enables the correction of up to (16−10)/2=3 corrupt bytes in the same byte positon (e.g., up to 3 corrupt bytes across the 16 slices when the bytes are all at the same offset position in each slice). In this example, having all 18 slices available would allow for the correction of up to 4 corrupt bytes. This approach can be used to attempt to restore a maximum number of corrupted byte positions in each offset in a given corrupt slice. After applying this process to modify the corrupt slices, level 3 integrity check information can be consulted to verify that the correction approach was successful in restoring corrupt slices.

If at least a decode threshold number of slices are determined to be valid (step 116) according to the level 3 integrity information, then the processing module attempts to dispersed storage error decode the slices in step 122. The decode operation can be verified using the level 1 or level 2 integrity information.

If the first correction approach is unsuccessful as determined in step 116, the processing module selects a second correction approach in step 118. The method then proceed to step 120 and the at least one corrupt encoded data slice is corrected using the second correction approach in order to produce a decode threshold number of valid encoded data slices. For example, if a level 1 or level 2 integrity error occurs after step 114 and all received slices are presumed to be valid, if there is at least one more slice than the decode threshold number of slices, the processing module may resort to a combinatorial decoding approach wherein different subsets of slices are excluded from the decode operation, and a decode and verification is attempted using level 1 and level 2 integrity check information. If no valid combination of slices can be identified using level 1 integrity check information, then a failure indication may be provided in response to the read request that initiated the method. If a combination of slices can be found which passes level 2 and level 1 integrity checks (as determined in step 122), the decoded data segment is returned to the requester.

With reference to FIG. 11, a combinatorial decoding approach might first utilize, for example, b1 of slices 1-4 and b1 of slice 5 in order to produce a first temporary decoding result. Additional temporary decoding results may be produced using b1 of slices 6 and 7. Comparison of the temporary results may then identify b1 of slice 6 as a corrupt byte. The temporary results of such a decoding approach may then be aggregated to recover an error free data segment.

If a valid data segment is decoded, the processing module may use it to determine which of the slices it received were valid and which were invalid. Invalid slices may be flagged for rebuilding, deletion, or rewritten to the storage units as valid slices. Even if write permission is not available for the storage units, a rebuild-write operation including valid slice integrity information may still succeed if, for example, a correct hash list is provided to the receiving storage unit and the storage unit verifies that the hash list is correct.

The methods described above in conjunction with the computing device and the storage units can alternatively be performed by other modules of the dispersed storage network or by other devices. For example, any combination of a first module, a second module, a third module, a fourth module, etc. of the computing device and the storage units may perform the method described above. In addition, at least one memory section (e.g., a first memory section, a second memory section, a third memory section, a fourth memory section, a fifth memory section, a sixth memory section, etc. of a non-transitory computer readable storage medium) that stores operational instructions can, when executed by one or more processing modules of one or more computing devices and/or by the storage units of the dispersed storage network (DSN), cause the one or more computing devices and/or the storage units to perform any or all of the method steps described above.

As may be used herein, the terms“substantially” and“approximately” provides an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from less than one percent to fifty percent. Such relativity between items ranges from a difference of a few percent to magnitude differences. As may also be used herein, the term(s)“configured to”, “operably coupled to”, “coupled to”, and/or “coupling” includes direct coupling between items and/or indirect coupling between items via an intervening item (e.g., an item includes, but is not limited to, a component, an element, a circuit, and/or a module) where, for an example of indirect coupling, the intervening item does not modify the information of a signal but may adjust its current level, voltage level, and/or power level. As may further be used herein, inferred coupling (i.e., where one element is coupled to another element by inference) includes direct and indirect coupling between two items in the same manner as“coupled to”. As may even further be used herein, the term“configured to”, “operable to”, “coupled to”, or“operably coupled to” indicates that an item includes one or more of power connections, input(s), output(s), etc., to perform, when activated, one or more its corresponding functions and may further include inferred coupling to one or more other items. As may still further be used herein, the term“associated with”, includes direct and/or indirect coupling of separate items and/or one item being embedded within another item.

As may be used herein, the term“compares favorably”, indicates that a comparison between two or more items, signals, etc., provides a desired relationship. For example, when the desired relationship is that signal 1 has a greater magnitude than signal 2, a favorable comparison may be achieved when the magnitude of signal 1 is greater than that of signal 2 or when the magnitude of signal 2 is less than that of signal 1. As may be used herein, the term“compares unfavorably”, indicates that a comparison between two or more items, signals, etc., fails to provide the desired relationship.

As may also be used herein, the terms“processing module”, “processing circuit”, “processor”, and/or“processing unit” may be a single processing device or a plurality of processing devices. Such a processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on hard coding of the circuitry and/or operational instructions. The processing module, module, processing circuit, and/or processing unit may be, or further include, memory and/or an integrated memory element, which may be a single memory device, a plurality of memory devices, and/or embedded circuitry of another processing module, module, processing circuit, and/or processing unit. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, cache memory, and/or any device that stores digital information. Note that if the processing module, module, processing circuit, and/or processing unit includes more than one processing device, the processing devices may be centrally located (e.g., directly coupled together via a wired and/or wireless bus structure) or may be distributedly located (e.g., cloud computing via indirect coupling via a local area network and/or a wide area network). Further note that if the processing module, module, processing circuit, and/or processing unit implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory and/or memory element storing the corresponding operational instructions may be embedded within, or external to, the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. Still further note that, the memory element may store, and the processing module, module, processing circuit, and/or processing unit executes, hard coded and/or operational instructions corresponding to at least some of the steps and/or functions illustrated in one or more of the Figures. Such a memory device or memory element can be included in an article of manufacture.

One or more embodiments have been described above with the aid of method steps illustrating the performance of specified functions and relationships thereof. The boundaries and sequence of these functional building blocks and method steps have been arbitrarily defined herein for convenience of description. Alternate boundaries and sequences can be defined so long as the specified functions and relationships are appropriately performed. Any such alternate boundaries or sequences are thus within the scope and spirit of the claims. Further, the boundaries of these functional building blocks have been arbitrarily defined for convenience of description. Alternate boundaries could be defined as long as the certain significant functions are appropriately performed. Similarly, flow diagram blocks may also have been arbitrarily defined herein to illustrate certain significant functionality.

To the extent used, the flow diagram block boundaries and sequence could have been defined otherwise and still perform the certain significant functionality. Such alternate definitions of both functional building blocks and flow diagram blocks and sequences are thus within the scope and spirit of the claims. One of average skill in the art will also recognize that the functional building blocks, and other illustrative blocks, modules and components herein, can be implemented as illustrated or by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof.

In addition, a flow diagram may include a“start” and/or“continue” indication. The “start” and“continue” indications reflect that the steps presented can optionally be incorporated in or otherwise used in conjunction with other routines. In this context, “start” indicates the beginning of the first step presented and may be preceded by other activities not specifically shown. Further, the“continue” indication reflects that the steps presented may be performed multiple times and/or may be succeeded by other activities not specifically shown. Further, while a flow diagram indicates a particular ordering of steps, other orderings are likewise possible provided that the principles of causality are maintained.

The one or more embodiments are used herein to illustrate one or more aspects, one or more features, one or more concepts, and/or one or more examples. A physical embodiment of an apparatus, an article of manufacture, a machine, and/or of a process may include one or more of the aspects, features, concepts, examples, etc. described with reference to one or more of the embodiments discussed herein. Further, from Figure to Figure, the embodiments may incorporate the same or similarly named functions, steps, modules, etc. that may use the same or different reference numbers and, as such, the functions, steps, modules, etc. may be the same or similar functions, steps, modules, etc. or different ones.

Unless specifically stated to the contra, signals to, from, and/or between elements in a figure of any of the figures presented herein may be analog or digital, continuous time or discrete time, and single-ended or differential. For instance, if a signal path is shown as a single-ended path, it also represents a differential signal path. Similarly, if a signal path is shown as a differential path, it also represents a single-ended signal path. While one or more particular architectures are described herein, other architectures can likewise be implemented that use one or more data buses not expressly shown, direct connectivity between elements, and/or indirect coupling between other elements as recognized by one of average skill in the art.

The term“module” is used in the description of one or more of the embodiments. A module implements one or more functions via a device such as a processor or other processing device or other hardware that may include or operate in association with a memory that stores operational instructions. A module may operate independently and/or in conjunction with software and/or firmware. As also used herein, a module may contain one or more sub-modules, each of which may be one or more modules.

As may further be used herein, a computer readable memory includes one or more memory elements. A memory element may be a separate memory device, multiple memory devices, or a set of memory locations within a memory device. Such a memory device may be a read-only memory, random access memory, volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, cache memory, and/or any device that stores digital information. The memory device may be in a form a solid state memory, a hard drive memory, cloud memory, thumb drive, server memory, computing device memory, and/or other physical medium for storing digital information.

While particular combinations of various functions and features of the one or more embodiments have been expressly described herein, other combinations of these features and functions are likewise possible. The present disclosure is not limited by the particular examples disclosed herein and expressly incorporates these other combinations. 

What is claimed is:
 1. A method for execution by one or more processing modules of one or more computing devices of a dispersed storage network (DSN), the method comprises: in response to a request for encoded data slices, receiving less than a decode threshold number of encoded data slices and at least one integrity error message from a set of storage units of the DSN in which the encoded data slices are stored, wherein each of the encoded data slices is an encoded portion of a corresponding data segment, and the at least one integrity error message provides an indication of a corrupt encoded data slice; requesting at least one corrupt encoded data slice from the set of storage units; receiving the at least one corrupt encoded data slice; and correcting the at least one corrupt encoded data slice, utilizing at least one correction approach, to produce at least the decode threshold number of encoded data slices.
 2. The method of claim 1, further comprising: decoding the decode threshold number of encoded data slices to generate the corresponding data segment.
 3. The method of claim 1, wherein correcting the at least one corrupt encoded data slice includes: selecting a first correction approach based on available integrity information; attempting to correct the at least one corrupt encoded data slice using the first correction approach; in response to determining that the first correction approach was not successful, selecting a second correction approach based on available integrity information; and correcting the at least one corrupt encoded data slice using the second correction approach.
 4. The method of claim 1, wherein the at least one correction approach includes applying stored integrity information corresponding to the at least one corrupt encoded data slice to determine if the data of the encoded data slice is valid.
 5. The method of claim 1, the at least one correction approach utilizing slice substitution and a stored integrity value corresponding to a pre-encoded version of the data segment.
 6. The method of claim 1, wherein the at least one correction approach includes byte-by-byte recovery of the at least one corrupt encoded data slice utilizing valid bytes, in a common byte position, from a decode threshold number of encoded data slices.
 7. The method of claim 1, the at least one correction approach utilizing at least one of: stored integrity information regarding the data segment, stored integrity information generated by a dispersed storage error encoding process during encoding of the encoded data slices, or stored integrity information regarding individual ones of the encoded data slices generated prior to storage of the encoded data slices in the set of storage units. 